Temperature responsive switch

ABSTRACT

The invention relates to a temperature responsive switch assembly of the type having a pivotally mounted lever attached to a snap action spring with the lever being pivotally movable in response to opposing forces provided by a rated spring and a vapor containing working element. The moving contact of the switch is attached to the lever. Although the vapor pressure curve of the working element is not linear, the temperature differentials between the switch on and off temperatures are kept substantially constant for the whole range of nominal temperature settings for the system. This object is achieved by an arrangement in which the line of force of the rated spring forms an acute angle with the lever which angle is of a selected magnitude that compensates for the nonlinearity of the vapor pressure curve of the working element is provided.

United States Patent 1 Andresen TEMPERATURE RESPONSIVE SWITCH [75]inventor: Jens Nicolai Andresen, V. Sottrup,

Denmark [73] Assignee: Danfoss A/S, Nordborg, Denmark [22] Filed: June8, 1972 [21] App]. No.: 261,491

[30] Foreign Application Priority Data Primary Examiner-Bernard A.Gilheany Assistant Examiner-D. A. Tone AttorneyWayne B. Easton 57ABSTRACT The invention relates to a temperature responsive switchassembly of the type having a pivotally mounted lever attached to a snapaction spring with the lever being pivotally movable in response toopposing forces provided by a rated spring and a vapor containingworking element, The moving contact of the switch is attached to thelever. Although the vapor pressure curve of the working element is notlinear, the temperature differentials between the switch on and offtemperatures are kept substantially constant for the whole range ofnominal temperature'settings for the system. This object is achieved byan arrangement in which the line of force of the rated spring forms anacute angle with the lever which angle is of a selected magnitude thatcompensates for the nonlinearity of the vapor pressure curve of theworking element is provided.

4 Claims, 4v Drawing Figures PATENTEDNBV BIHIS v 7 3771090 SHEET 1m 2TEMPERATURE RESPONSIVE SWITCH The invention relates to a temperatureresponsive assembly of the snap action type. The assembly comprises arigid lever, a snap action spring, a vapor containing working elementand an adjustable rated-value spring which counteracts the workingelement.

In switches of the kind here considered, the snap action spring inparticular a spring having the shape of the Greek letter omega, has anegative spring characteristic in the working direction. The snap overaction therefore takes place when the difference between the forcesapplied by the working element and the ratedvalue spring exceed theforce of the snap action spring in the rest position. These switches aretherefore called force-proportional springs in contrast todistanceproportional springs in which at least one lever of the snapaction system must be moved through a dead-cent position before thesnap-over action takes place.

Since the vapour pressure curve is not linear, the temperaturedifference, i.e., the difference between the switch-on temperature andthe switch-off temperature varies with each nominal temperature to whichthe system is set. It is however desirable to keep this differenceapproximately constant over the entire working range. It is thereforeknown practice in the case of temperature responsive actuating devicescomprising a vapor containing working element and a rated value spring,so to design this spring that the steepness of the spring characteristiccurve increases as the spring force rises, so that the springcharacteristic curve resembles the vapour pressure curve. In the case offorce proportional temperature responsive switches, compensation cannotbe achieved in this way.

A force proportional temperature responsive switch is known in which useis made of a double-armed lever. The working element and the rated valuespring are arranged side-by-side, one on each side of the point ofrotation, and act on the lever over an angle of 90.

In another force proportional switch,-the working el-' ement and therated-value spring are arranged coaxially with each other. They engage acommon rod. The rod transmits the difference in force to the lever.

The object of the present invention is to provide a force proportionaltemperature responsive switch which is at least partially compensated.

According to the invention this object is achieved by an arrangement inwhich the line along which the rated value spring applies force formswith the lever an angle that deviates from 90 to an extent such that theeffect of the bend in the vapour pressure curve on the difference in theswitching temperatures is at least partially compensated.

Such compensation is achieved if the line along which force is appliedby the rated value spring forms an angle of less than 90 with thatportion of the lever presented to the point of rotation.

It has proved particularly advantageous if the angle between the linealong which the force of the rated value spring is applied and the leveris 60 80, and preferably 65- 75.

If the line along which the rated value spring applies force is inclinedin the manner described, a very surprising result is that the steepnessof the characteristic curve of the spring force acting on the leverdecreases as said force increases. A curve of this kind, however, isprecisely what is required in a force proportional switch in order tocompensate at least partially the curvature in the characteristic curveof the spring. Conversely, the aforementioned slope of the line alongwhich the force of the working element is applied leads to the curve ofthe vapor pressure, that acts on the lever, becoming less pronounced.

A particularly space saving construction is obtained if the leverconsists of two arms which form an obtuse angle with each other, and ifthe rated value spring engages that arm remote from the snap actionspring and extends substantially parallel with the arm presented to thesnap action spring. The switch can then be formed as a very compact unitby fitting the working element and the contact system side-by-side onthat side of the arm presented to the snap action spring that isopposite thev rated value spring.

The invention will now be described in greater detail by reference tothe attached drawing, in which:

FIG. 1 is a diagrammatic illustration of a form of construction inaccordance with the invention.

FIG. 2 is a graph of the vapour pressure curve and the forces occurringat the springs,

FIG. 3 is a longitudinal section through a temperature responsive switchin accordance with the invention, and

FIG. 4is a cross section on the line A-A of FIG. 3.

The temperature responsive switch comprises a snapaction system whichconsists of a double-armed lever l and a spring 2 of the shape of theGreek letter omega. The lever is adapted to rotate about a pivot 3 solidwith the casing. It forms a link 4 with the spring 2. This spring issecured on a backing element 5 which is solid with the casing and whichmay be adjustable. The lever consists of two arms 6 and 7. The force Pdof a working element is applied perpendicularly to the arm 6. The forceP of the rated value spring is applied at an angle a of 60 80 andpreferably The rated value spring is a tension spring which can beelongated in the direction of its force line. Mounted on the lever arm 6is an electric contact 8 which cooperates with a fixed contact 9. Inthis arrangement this latter contact also acts as a mechanical stop. Theat-rest position is determined by a stop 10 solid with the casing.Consequently, when the snap-over action occurs, the lever 1 can swingthrough an angle 3,.

Onthe left of FIG. 2, the vapour pressure P,, of the working element isplotted against temperature; on the right are illustrated the forcesoccurring in the snapaction system at different positions and fordifferent settings of the rated value spring. The force curve is shownagainst the angle of swing B. The range of swing is limited to the angle3,. The omega spring 2 has a negative characteristic curve c in thisrange. For a first setting of the rated value spring giving a smallforce P,, a force having the characteristic curve c and acting on thelever l is obtained in the range of swing, and for a greater force P thecharacteristic curve c, is obtained. If the effective characteristiccurve for the rated-value spring and the characteristic curve for theomega spring are added together, then for the first setting (small forceP the characteristic curve c, is obtained and for the second setting(large force P,), the characteristic curve c, results. The forcesdetermined by these characteristic curves counteract the force of thevapor pressure curve P,,. If, for the first setting, the vapour pressuremoves up past to the point a the system snaps over and the working.point moves from b to 0. Only when the vapour pressure has droppedbelow the point d does the system snap back into the initial position.Similar conditions are obtained with the second setting. When the pointe is exceeded, the system snaps from f to g. It only snaps back againwhen the vapour pressure falls below point h.

It can be seen from the graph that the steepness of the springcharacteristic curves and c acting on the lever diminish as the springforce increases. Consequently, when the spring force is small, a smallforce difference D results, and with a large force, a large forcedifference D is obtained. Consequently, the temperature differences Aand A, at which change-over occurs are approximately constant despitethe bend in the vapour pressure curve.

The effective spring characteristic curve (steepness decreasing as forceincreases), here of interest, is obtained by causing the rated valuespring to apply force to the lever 1 along a line that is at an angle ofa to the lever. Although in this arrangement the force of the ratedvalue spring which counteracts the force P varies in proportion to itselongation, the rise in the characteristic curve that takes place in theangle of swing B surprisingly does not remain constant, but decreases asthe force increases.

FIGS. 3 and 4 illustrate a practical form of construction of thetemperature responsive switch of the invention. In these figures thesame reference numerals as in FIG. 1 are used as far as possible forcorresponding parts. The lever 1 having two arms 6 and 7 is fitted in acasing 11. The lever 1 simply rests on the bearing 3 solid with thecasing, and is drawn towards the fixed bearing 3 with the aid of therated value springs which take the form of tension springs 26. The ratedvalue springs 26 are held at their other, ends by backing elements 12.For the purpose of carrying out the initial adjustment, these backingelements are mounted on screw threaded members 13. The latter membersare secured to a slide 14 which can be displaced by means of a cammeddisc 15 on an adjusting spindle 16 for the purpose of setting to therequired value. I

A working element 17 is formed by a tubular bellows 18 and a part 19 ofthe casing. A capillary tube 20 communicates with a sensor, notillustrated. The temperature of the sensor determines the vapourpressure in the working element 17. The end face of the tubular bellowsl8 acts on a raised portion 21 on the lever arm 6. In this arrangementthe first step is not formed by a fixed contact 9 but by a part 22 ofthe casing, which part surrounds a contact system 23. As shown in FIG.4, the lever arm 6 acts on a plunger 24 which in turn acts on a spring25 carrying the movable contact of the system 23.

FIG. 4 parallel shows that two rated-value springs are arrangedside-by-side and aprallel with each other, and that the stops 10 aredisposed symmetrically, one on each side of a line of symmetry.

Instead of the omega spring, another type of snap action spring may beused that has a negative characteristic curve in the working range. Theidea underlying the invention may also be applied to snap action systemshaving a single anned lever, and to such levers in which the lines alongwhich the forces of the working element and the rated value spring runto the same point on the lever.

I claim:

1. A temperature responsive switch assembly comprising a casing, a rigidlever in said casing, pivot means for mounting said lever for pivotalmovement relative to a fixed part of said casing, a snap action springbetween said lever and said casing, a working element positioned betweenand connected to said casing and said lever, said working element havinga moving part for applying forces to said lever on one side of saidpivot means, said working element having a nonlinear vapor pressurecurve, spring means attached to said casing and said lever for applyingforces to said lever which oppose said working element applied forces tosaid lever, said spring means forces being applied to said lever on theside of said pivot means opposite said one side,

said spring means having a line force forming an angle with said leverso that the reaction response of said I spring means parallels saidworking element nonlinear vapor pressure curve.

2. A temperature responsive switch assembly according to claim 1 inwhich said lever has two arms forming an obtuse angle with the apexthereof concident with said pivot means, said working element being incontact with one of said lever arms, said spring means being attached tothe other of said lever arms with said I other of said arms.

1. A temperature responsive switch assembly comprising a casing, a rigidlever in said casing, pivot means for mounting said lever for pivotalmovement relative to a fixed part of said casing, a snap action springbetween said lever and said casing, a working element positioned betweenand connected to said casing and said lever, said working element havinga moving part for applying forces to said lever on one side of saidpivot means, said working element having a nonlinear vapor pressurecurve, spring means attached to said casing and said lever for applyingforces to said lever which oppose said working element applied forces tosaid lever, said spring means forces being applied to said lever on theside of said pivot means opposite said one side, said spring meanshaving a line force forming an angle with said lever so that thereaction response of said spring means parallels said working elementnonlinear vapor pressure curve.
 2. A temperature responsive switchassembly according to claim 1 in which said lever has two arms formingan obtuse angle with the apex thereof concident with said pivot means,said working element being in contact with one of said lever arms, saidspring means being attached to the other of said lever arms with saidline of force thereof being at an acute angle relative to said one ofsaid arms.
 3. A temperature responsive switch assembly according toclaim 2 wherein said acute angle is within the range of 65* to 75*.
 4. Atemperature responsive switch assembly according to claim 2 wherein saidspring means engages the one of said arms which is remote from said snapaction spring, said spring means being generally parallel to the otherof said arms.